The present disclosure relates to direct gas-fired heated ventilation systems for industrial and commercial buildings, and particularly to systems which use variable amounts of fresh air.
More specifically, the present disclosure relates to a constant pressure self adjusting burner bypass for such direct gas-fired heated ventilation systems.
Direct gas-fired heated ventilating systems provide heated make-up air to buildings by drawing fresh air into them then exhausting it into a building. Such make-up air is necessitated by the loss of heated inside air through various exhaust fans and chimney flues. The quantity of heated replacement air is usually substantially equal to the exhausted heated air. To prevent infiltration of cold outside air, it is also common practice to provide slightly more make-up air than is lost so that a slight positive pressure is maintained inside the building. The operation of the heated ventilation system is controlled to maintain a selected inside air temperature and a desired building inside/outside air pressure differential. This type of heated ventilation system is commonly called a make-up air unit. Generally, it is not permitted to use a direct gas-fired heated ventilating systems to heat a building because the combustion gases are introduced into the building. It can only be used to heat replacement air.
There are two basic types of direct gas-fired make-up air units (DGF make-up air unit). A first basic type includes a fresh air intake which communicates with a chamber containing a direct gas-fired burner. The burner heats the air as it is drawn or blown over the flame by a blower fan. The heated fresh air and the combustion gases are exhausted into the building. In some of such systems, the amount of air being exhaust out of the building may vary requiring that the make-up air also vary. In some systems, the rotational speed of the blower fan is controlled to vary the amount of heated make-up air generated, and in other systems the blower fan is turned on and off as needed.
An inherent characteristic of known DGF make-up air units is that the airflow velocity at the burner must remain constant to maintain optimum combustion chemistry. To allow variable quantities of heated air to be introduced into the building, the quantity of air heated by the burner is made constant and a variable quantity of replacement air is then shunted passed the burner unheated and then mixed with the fixed quantity of heated air such that the resulting mixture will be at the desired temperature. Example systems of this type are disclosed in U.S. Pat. No. 3,591,150, issued on Jul. 6, 1971 to Weatherston and titled “Gas Furnace” and U.S. Pat. No. 4,325,352 issued on Apr. 20, 1982 to Dirkes and titled “Internal Recirculation Device”.
The second basic type of system recirculates inside air and mixes it with the heated fresh air before delivering it to the inside space. These systems include a recirculating air intake which draws air from inside the building and mixes it with the heated fresh air. With this system type, the airflow through the replacement air blower fan remains constant. Dampers control the relative amounts of each air stream. Examples of this type of direct gas-fired air make-up units are disclosed in the following references:    U.S. Pat. No. 3,417,977, issued on Dec. 24, 1968 to Nelson and titled “Air Control System for Heating Unit”;    U.S. Pat. No. 4,429,679, issued on Feb. 7, 1984 to Dirkes and titled “Modular Air Heater”;    U.S. Pat. No. 4,573,912, issued on Mar. 4, 1986 to Albritton et al. and titled “Space Heater”;    U.S. Pat. No. 4,674,475, issued on Jun. 23, 1987 to Powis and titled “Gas Fired Furnace”; and    U.S. Pat. No. 4,917,074, issued on Apr. 17, 1990 to Brekke and titled “Direct gas-Fired Heating and Ventilation Method and Apparatus”.
As mentioned above, the air stream over the direct gas-fired burner is maintained at a constant velocity for optimum combustion. This implies that the pressure drop across the burner be maintained at a substantially constant level. In the above-cited patents this is achieved by controlling dampers that supply air to the burner or dampers that shunt air around the burner. In all cases, the dampers are either controlled by complex interlocking mechanisms, linkages with springs or counterweights or are separately controlled by electronic controllers. Such control methods are disclosed in the following documents:    U.S. Pat. No. 4,829,447, issued on May 9, 1989 to Parker et l. and titled “Bypass Controller and Bypass System”;    U.S. Pat. No. 5,257,958, issued on Nov. 2, 1993 to Jagers and titled “Pressure Override Control for Air Treatment Unit”;    U.S. Pat. No. 5,597,354, issued on Jan. 28, 1997 to Janu et al. and titled “Indoor Air Quality Control for Constant Volume Heating, Ventilating and Air Conditioning Units”;    A known manufacturer of make-up air units, CaptiveAire of Raleigh, N.C., USA, employs vertical spring biased blades on both side of the burner.